1. Field of the Invention
The present invention relates to a process and an apparatus for building a pneumatic tyre.
2. Description of the Related Art
Generally, a tyre for vehicle wheels has a carcass structure essentially made up of one or more carcass plies substantially having a toroidal conformation. The axially opposite side edges of these plies are engaged to respective annular reinforcing structures provided with at least one bead core.
Each annular reinforcing structure is incorporated into a so-called “bead” defined along an inner circumferential edge of the tyre for anchoring the latter to a corresponding mounting rim.
Generally associated with the carcass structure is also a belt structure comprising one or more layers disposed in radial superposed relationship with respect to each other and to the carcass ply.
The belt layers generally consist of textile or metallic reinforcing cords with a crossed orientation and/or substantially parallel to the circumferential extension direction of the tyre. Applied to the belt structure at a radially external position is a tread band, made of elastomeric material like the other semifinished products constituting the tyre.
To the aims of the present description, by the term “elastomeric material” it is intended a composition comprising at least one elastomeric polymer and at least one reinforcing filler, and possibly additives of various types (cross-linking agents, plasticizers and other additives known in the art, for example).
Finally, a pair of sidewalls is provided on the axially opposite sides of the tyre, each sidewall coating a side portion of the tyre included between a so-called shoulder region, located close to the corresponding side edge of the tread band, and the corresponding bead.
In traditional manufacturing processes, formation of the carcass ply is carried out through manufacture of a rubberized fabric comprising a plurality of reinforcing thread elements, parallel to each other and incorporated into a layer of elastomeric material, generally referred to as “cords”. Said rubberized fabric is for example made through a calendering operation sandwiching a plurality of said cords, disposed coplanar in side by side relationship, between two layers of elastomeric material.
The threads of said cords can consist of metal alloys such as steel, textile fibres, carbon fibres, glass fibres, Kevlar® or other fibres having a suitable strength for use in tyre construction.
Each carcass ply is then assembled to the other tyre components such as the above mentioned bead cores, on a so-called cylindrical building drum. The carcass ply is turned up around said bead cores in such a manner that when the whole carcass structure, initially made in the form of a cylindrical sleeve, is shaped into a toroidal configuration, the carcass ply and bead cores remain associated with each other, with the carcass ply turned up around said bead cores, being axially and radially extended from the inside to the outside.
Then in a subsequent operation, other tyre components such as the belt structure, sidewalls and tread band, are assembled on said carcass structure.
Referring again to the operation of turning up the carcass ply around the respective annular reinforcing structures, the particular critical character of this step must be herein emphasised. In fact, a bad turning up gives rise to structural problems in the tyre resulting from a partial slipping off of the carcass ply from the annular reinforcing structures, during manufacture of the tyre itself.
To meet the requirement of making carcass structures more linked to the annular reinforcing structures and to obviate the above mentioned drawbacks, some different solutions have been proposed.
For instance U.S. Pat. No. 6,318,434 discloses a tyre building drum with a turning-up device for a carcass ply of the traditional type in which the drum has two annular segments spaced apart from each other to support a bead core, and drum segments disposed inside each annular segment for the purpose of supporting some tyre components. The building drum has means for radially expanding that part of the type components supported by the inner drum segments.
In addition, the drum on both sides external to the annular segments has a first and a second set of axially extensible arms. Each arm has one end provided with a roller, and means for axially and radially moving each set of arms from a first position at which the rollers of one set form a virtually closed loop and a second radially expanded position at which, during the turning-up step, pressing of the expanded part of the tyre components that is located externally of the annular segments is carried out. Each roller of an arm of the second set is placed between two adjacent arms of the first set.
U.S. Pat. No. 3,887,423 discloses a tyre-manufacturing machine using a turning-up mechanism for carcass plies that is disposed sideways, at each end of a building drum. The mechanism comprises a plurality of arms pivotally mounted to one end and an annular expandable rubber air bag provided on said arms. The free end of each arm is equipped with a roller such disposed that it comes into engagement with the inside of the air bag. The arms rotate about their hinging point to turn the carcass ply up and are axially movable to close the carcass ply upon itself, when turned up.
U.S. Pat. No. 4,362,592 discloses a ply turning-up device for use with a building drum. The turning-up device comprises a plurality of arms disposed so as to form a circumference concentric to the axis of the building drum. The arms can be extended radially outwardly and have a radially movable free end to support a ring-shaped rotatable and expandable element. The latter is embodied by an annularly continuous helical spring. To radially expand the annular element, each arm has a cam follower co-operating with a cam disposed concentric to the drum axis.
The Applicant has first of all noticed that with a device like the one disclosed in U.S. Pat. No. 6,318,434 the tyre thus obtained may have faults due to a defective adhesion of the turned-up ply portions along the whole application circumference. This is essentially due to the impossibility of a circumferentially continuous and uniform pressure being obtained on the turned-up ply portion.
The Applicant has further noticed that with a device like that disclosed in U.S. Pat. No. 3,887,423, pressure is exerted in a circumferentially continuous manner, but not uniformly because this pressure varies between the air bag regions where the roller is present and those where said roller is absent.
The Applicant has also noticed that in a turning-up device of the above described type the air bag can be subject to quick wear so that frequent interventions for maintenance or replacement are required.
In addition, the Applicant has noticed that in a device like that disclosed in U.S. Pat. No. 4,362,592 the arm stroke for expansion of the annular element is absolutely determined by the inclination of the cam that is purposely designed depending on the size of the tyre to be manufactured. Therefore construction of a particular support drum is required, which drum must be provided with one turning-up device for each tyre size to be obtained.
Herein and in the following for tyre size it is intended the nominal fitting diameter of the rim, generally expressed in inches.
The Applicant has further noticed that according to the teachings of the known art the operation of bending the carcass plies is carried out with devices different from those causing turning-up around the annular reinforcing elements.
The Applicant has perceived that by exerting a circumferentially continuous and uniform pressure on the turned-up carcass ply portions, the drawbacks previously found on the tyre can be overcome, and has also observed that by varying the circumference sizes of the elements exerting pressure on the turned-up ply portions, tyres of different sizes can also be manufactured using the same apparatus.